Axis shift analysis of electrocardiogram signal parameters especially applicable for multivector analysis by implantable medical devices, and use of same

• US 20020016548A1
• Filed: 10/01/2001
• Published: 02/07/2002
• Est. Priority Date: 03/29/1999
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Nelson, Shannon, Stadler, Robert W.
• Granted Patent: US 6,397,100 B2
• Time in Patent Office: Days
• US Class Current: 600/509
• CPC Class Codes: A61B 5/341 Vectorcardiography [VCG]
• CPC Class Codes: A61B 5/349 Detecting specific parameters of the electrocardiograph cycle
• CPC Class Codes: A61B 5/358 Detecting ST segments
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61N 1/37 Monitoring; Protecting
• CPC Class Codes: Y10S 128/901 Suppression of noise in electric signal

Abstract

We show how to determine whether there has been an axis shift in an electrocardiogram waveform and how to use this for filtering out bad electrocardiogram information and to modify an adaptive filter that can be used to adapt the filtering of such electrocardiogram information to make it available for determining physiologic conditions even after an axis shift.

Claims

1. A method for determining whether there has been an axis shift in an electrocardiogram waveform comprising a repeated cycle of steps, each cycle comprising the steps:

a) for a given cardiac cycle, collecting into a buffer selected characteristic portions of the electrocardiogram signal and sampling specific point locations within said portions based on measured values of said samples for said portion collected, wherein a location corresponding to the one of said samples that corresponds in time to a fiducial point wherein said waveform,

b) determining a selection of samples to take from a cardiac cycle surrounding temporally said fiducial point and from and based on values of electrocardiogram amplitude of said samples producing a parameter representative of said a cardiac cycle,

c) determining if said parameter is out of a predetermined range of said parameter, and

d) declaring an axis shift if said parameter is out of range for more than a predetermined amount.

2. The method of claim 1 wherein said predetermined amount in step d comprises a predetermined number of times out of a predetermined number of cycles.

3. The method of claim 1 wherein said fiducial point is an R-peak for said a cardiac cycle, and the following steps are used to determine the R-peak point;

finding a peak positive value and a peak negative value in said buffer of samples,

finding an isoelectric sample,

employing the amplitude value of said isoelectric sample and said peak values to determine which of said peak values is furthest from said isoelectric sample value,

declaring the sample with the peak value further from said isoelectric sample value as the sample located at said R-wave peak.

4. The method of claim 3 wherein in step “

b”

said parameter value is an amplitude of an R-wave and is determined based on said R-wave peak value.

5. The method as set forth in claim 1 wherein said declaration of said parameter axis shift is used to determine ranges for acceptable values for determining a physiologic characteristic from the set of physiologic characteristics including T wave height variation, ischemic condition or QT variation.

6. The method of any of claims 1-3 further comprising the step of simultaneously performing steps a-d on a plurality of electrocardiogram vectors, and still further comprising the step of determining an axis shift combination value based on whether an axis shift is declared for more than a predetermined number of electrocardiogram vectors.

7. The method of any of claims 1-5 and further comprising the step of adjusting an alarm threshold for a physiologic parameter change based on whether an axis shift is declared.

8. The method of any of claims 1-5 further comprising the step of adapting baseline values of said measured parameter to change responsive to declaration of an axis shift.

9. The method of any of claims 1-5 further comprising the step of adapting ST change variable filter coeficients responsive to declaration of an axis shift.

10. The method of any of claims 1-5 further comprising the step of adapting a noise filter to reduce sensitivity to motion artifacts in a group of cardiac cycles surrounding the temporal location of a declaration of an axis shift.

11. Apparatus for determining if an axis shift has occurred in an electrocardiogram signal so as to provide a better data set for evaluating a physiologic condition based on said cardiac electrogram signal, wherein said apparatus receives said electrocardiogram signal through a plurality of electrical leads connected to provide said cardiac electrogram signal from a plurality of electrical vectors, said apparatus comprising:

an electrogram reading device connected to said plurality of electrodes for sensing the amplitude variation in the electrical signal of a heart for each electrical vector,

a sampling and digitizing circuit for digitizing samples of said cardiac electrical amplitude signal and to provide an output stream of digitized sample point values representative of said samples,

a V-event detection circuit for generating a V-event signal at its output indicating that a ventricular event has occurred,

a buffer circuit for holding a set of point values that are temporally related to a time in a cardiac cycle when said V-event signal is output from said V-event detection circuit,

a fiducial point determining circuit means for reading the set of point values from said buffer circuit for determining which of said point values is a fiducial point related to said V-event signal,

a subset determining and selecting circuit for determining based at least in part on a temporal relative location of said determined fiducial point to said set of point values which subset of said set of point values to select and selecting them,

a parameterizing processor circuit for producing a set of parameter values related to said set of point values selected and determined by said subset determining circuit wherein at least some of the parameters produced are evaluatable by a physiologic condition signal processor circuit,

a physiologic condition signal circuit means for calculating a physiologic condition signal variable value,

an expected range value recalculating circuit for employing said set of parameter values and said pre-calculated expected ranges to produce new pre-calculated expected ranges to produce new pre-calculated expected range values for a next set of parameter values to be compared with a comparison circuit for comparing said pre-calculated expected ranges with each parameter value from said set of parameter values to determine if it is within its expected range and producing a validity value and as output and for setting an axis shift flag value as an additional output if said parameter values exhibit axis shift characteristics,

a cardiac cycle validity determining circuit which recieves said validity value and said axis shift flag values for determining if said validity value is sufficient to employ said set of parameter values for determining a physiologic condition and for recalculating said set of expected ranges by said recalculating circuit and for providing a cycle validity flag value signal representing the determination of sufficiency of cycle validity for a cardiac cycle,

an evaluation circuit for evaluating said physiologic condition based on running changes in said current physiologic condition variable value from those cycles determined to be sufficiently valid.

12. Apparatus as set forth in claim 11 wherein said fiducial point is an R peak.

13. Apparatus as set forth in claim 12 wherein said expected range value recalculating circuit determines a value for said axis shift flag based on a value of said R peak.

14. Apparatus as set forth in claim 11 wherein said sampling and digitizing circuit comprises circuit means for sampling a plurality of vectors of said electrogram signal simultaneously.

15. Apparatus as set forth in claim 13 wherein said parameterizing processor circuit for producing a set of parameter values related to said set of point values selected and determined by said subset determining circuit produces a set of said parameter values for each vectorl

16. Apparatus as set forth in claim 11 wherein said apparatus further comprises a sealed and implantable housing having said electrodes mounted thereon for use within a living body, and further comprising a telemetry system for transmitting said parameters outside said living body.

17. Apparatus as set forth in claim 11 further comprising alarm threshold circuit means responsive to a predeteremined alarm value for said physiologic condition signals to generate an alarm signal if said apparatus determines a physiologic condition signal to have reached said predetermined alarm value, and having adaptive threshold setting circuit means for automatically adjusting said predetermined alarm value based on a change in said axis shift flag value.

18. Apparatus as set forth in claim 11 further comprising a noise supression circuit means responsive to a change in said axis shift flag value to adjust noise thresholds to allow for a larger range of signal amplitude in the presence of a percieved axis shift.

19. Apparatus as set forth in claim 11 further comprising adaptive filter circuit responsive to a change in said axis shift value for adjusting said filter circuit to pass an adjusted range of said physiologic condition signals to generate a secondary physiologic condition signal based on said physiologic condition signals passed through said adaptive filter circuit.

20. Apparatus as set forth in claim 11 further comprising adaptive baseline value adjustment circuit responsive to a change in said axis shift value for adjusting a baseline value of said physiologic condition signal to produce a signal for enabling adjustment of a drift filter parameter to accomodate axis shifts.